1. Technical Field
The present invention relates to a timepiece having a configuration in which a striking member strikes a struck member.
2. Related Art
Chronograph timepieces having an analog display using hands to indicate normal time also have chronograph hands such as a seconds chronograph hand and a minute chronograph hand to indicate chronograph time. The chronograph hands are made to start and stop keeping chronograph time, and are reset to zero, when operating buttons disposed to the timepiece are operated appropriately. The operation that returns the chronograph hand from where it stops to the original zero position (“return-to-zero” operation) is accomplished by pressing a flyback lever against a heart-shaped cam (heart cam) affixed to the staff on which the chronograph hand is disposed. See, for example, International Patent Application No. WO/1999/054792 and Japan Registered Utility Model U2605696.
In a chronograph timepiece such as described above the flyback lever presses against the heart cam. The flyback lever is thus a striking member and the heart cam is a struck member, and impact stress is produced by the collision between the flyback lever and the heart cam. More specifically, the staff becomes a striking member, the bearing becomes a struck member, and impact stress is produced between the staff and the bearing.
The impact stress produced by the collision between the striking member and the struck member can result in damage to or deformation of timepiece parts including the striking member and the struck member.
Considering in particular the bearing, that is, the struck member, if the bearing is made from a material with high hardness such as ruby, fractures may occur during use for a long period of time as a result of repeated impact between the staff and the bearing. Such fractures can be avoided by using a metal bearing, but the durability of metal bearings is poor, and wear from rotation of the staff and impact during the flyback operation can eventually deform the bearing. Yet, further, common metals that can improve resistance to wear and deformation are difficult to process and are not suited to mass production of timepieces.